Transcript for Trayvon Martin's Dad: 'My World Has Been Turned Upside Down'

the trial of george zimmerman. Gripping and emotional testimony on monday. The defense arguing that something he says bolsters the case for zimmerman's innocence. Abc's matt gutman has that story. Reporter: For the first time, trayvon martin's grieving father on the stand. Our world has just been turned upside-down. Tracy martin telling jurors that these pleas for help. Do you think he's yelling help? Reporter: Heard in the background of this 911 call, were his son screaming. I was listening to my son's last cry for help. I was listening to his life being taken. Reporter: Prosecutors never called martin to the stand. But on monday, the attorneys defending george zimmerman did, suggesting martin had changed his story to help the state's case. After initially telling police the voice he heard was not trayvon's. I never said, no, that wasn't my son's voice. I kind of shook my head and said, I can't tell. Reporter: Moments earlier, lead police investigator chris s serino, said he gave a different answer. He looked away. And under his breath, as i interpreted, said no. Reporter: Just who was crying for help has become the focus of much of the trial. The prosecutors believe the pleas for help came from 17-year-old martin. But zimmerman's lawyers brought a parade of friends on the stand saying the voice belonged to him. It was georgey. The voice, obviously changes. I've heard a 250-pound man, i mean, sound like a little girl. Reporter: Zimmerman has pleaded not guilty to second-degree murder, claiming martin attacked him and he shot in self-defense, after those screams for help for ignored. Adam pollack, the man who trained zimmerman in kick boxing, told jurors he was not a fighter. He didn't know how to effectively punch. He was predominantly fat. Not a lot of muscle. Reporter: But legal analysts say that's not justification for zimmerman pulling the trigger. He did testify that george zimmerman was a cream puff. But the cream puff had a gun. Reporter: This morning, we expect the focus to shift from those phone calls to forensics. A noted pathologist is going to be put on the stand. The defense has put 12 witnesses on the stand so far. They're flying through it. It is possible they could wrap up their case as early as today. The jury could get this case as early as the end of the week. Robin? We'll see what happens. Matt, thank you. We're going to bring in abc's chief legal affairs anchor, dan abrams. A lot of attention, a lot of emphasis on the 911 call. Tracy martin, trayvon's father, so emotional. How is this playing to the jury? The defense is calls witness after witness to say it was george zimmerman. Even if this jury isn't sure, that's a win for the defense. That's reasonable doubt. So, I think that's all the defense is really trying to do here. Calling tracy martin was a risky move. There's two police officers testified that the first time tracy martin heard that type, he did not think it was his son, tr trayvon. We have the police officers' testimony. They wanted to call tracy martin. In conjunction with getting an effort to undermine tracy martin, you get the very emotional, powerful side of tracy martin, the dad, testifying in effect on behalf of trayvon. And today, we heard him answer for it. And things continue, with a forensic expert. What do you think we're going to hear today? I think we're going to hear about george zimmerman's injuries and the gunshot wounds. The defense trying to show this is defensive. The defense wants to re-create, create a 3d recreation on how this could have happened. The judge hasn't ruled on how much of that she's going to let in. But that's how I think the defense wants to wrap all this up. They want to say, this is how we believe it could have occurred. Matt said moving along quickly here. All along, you have been -- you have said, this is a tough case for the prosecution. Yeah. And you still feel that way? Yeah. After the prosecution's case, i believe it's hard to imagine, how based on the evidence, that the jurors have seen, they can convict as a legal matter on manslaughter or second-degree murder. But juries are impossible to predict. They make the decisions. And they sometimes choose to go their own way. There's others who say they wouldn't be ignoring the law. The jurors have heard every, single moment of testimony. That's right. We'll see what happens this week. All right.

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